Gyros have been used to measure rotation rates or changes in angular velocity about an axis of rotation. A basic conventional fiber optic gyro (FOG) includes a light source, a beam generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. Many FOGs utilize glass-based optical fibers that conduct light along a solid glass core of the fiber. The two counter-propagating (e.g., CW and CCW) beams may experience different pathlengths while propagating around a rotating closed optical path, and the difference in the two pathlengths is proportional to the rotational rate.
In a resonator fiber optic gyro (RFOG), the counter-propagating light beams are desirably monochromatic (e.g., in a single frequency) and circulate through multiple turns of the fiber optic coil and for multiple passes through the coil using a device, such as a fiber coupler, that redirects light that has passed through the coil back into the coil again (i.e., circulates the light). The beam generating device modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The resonance frequencies for each of the CW and CCW paths through the coil are based on a constructive interference condition such that all light-waves having traversed the coil a different number of times interfere constructively at any point in the coil. As a result of this constructive interference, an optical wave having a wavelength λ is referred to as “on resonance” when the round trip resonator optical pathlength is equal to an integral number of wavelengths. A rotation of the coil produces a different optical pathlength for clockwise and counterclockwise propagation, thus producing a shift between the respective resonance frequencies of the resonator, and the frequency difference, such as may be measured by tuning the difference in the CW beam and CCW beam frequencies to match the resonance frequency shift of the closed optical path due to rotation, indicates the rotation rate.
Frequency shifters and phase modulators have been used with the beam generating device (e.g., a laser light source) to modulate and shift the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The frequency shifters and phase modulators may be difficult to economically implement particularly for smaller scale applications or implementations of the resonator gyro. Alternatively, tunable lasers may be used for introducing the counter-propagating light beams and for resonance detection without the use of frequency shifters and phase modulators. However, when using two or more tunable lasers for resonance detection, the relative frequency noise between such lasers is typically the greatest contributor to angle random walk (ARW) which decreases the rotation rate accuracy of detected by the resonator gyro.
In the RFOG, the glass material of the optical fiber may give rise to effects that shift the resonance frequencies of the CW and CCW paths and thus produce a false indication of rotation or inaccurate measurement of rotation rate. Anomalies stemming from the glass medium that decrease the accuracy of the measurement of the rotational rate may be generated from a non-linear Kerr effect, stimulated Brillouin scattering (SBS), polarization errors, and Rayleigh backscatter errors. These error mechanisms are also sensitive to the environment which, for example, gives rise to unwanted temperature sensitivity. A reflective mirror may be used to circulate the counter-propagating light beams in the coil multiple times but this typically reduces the signal-to-noise ratio.
The non-linear Kerr effect occurs when high monochromatic light power inside the RFOG alters the index of refraction of the glass in the optical fiber. A mismatch of intensities of the CW and CCW beams may induce a bias on the observed frequency shifts on the order of several degrees/hour. SBS occurs when a high intensity associated with a high finesse in the fiber resonator causes lasing or stimulated emission in the glass fiber, and this generally promotes large instabilities in the measurement of the resonance frequencies. Polarization-induced errors may result from fiber couplers that incidentally couple light into a second polarization mode, either from one optical fiber to an adjacent optical fiber or within the same fiber. The second polarization mode may resonate producing an asymmetry in the resonance lineshape of the polarization mode used to measure a rotation. Even though the frequency of the second polarization mode is the same for the CW and CCW beams, the amplitude may be different, thus causing different observations, beyond the affect of rotation, of the resonance frequencies of the CW and CCW beams. Polarization-induced errors may severely limit the accuracy of the RFOG because determination of the resonance centers for each of the resonance frequencies of the CW and CCW beams directly affects the rotational rate measurement. Rayleigh backscatter errors may be a source of drift rate in a resonator gyro. Backscattered light from the glass within the fiber or from imperfections with the fiber can interfere with the circulating light beams and cause significant drift rate errors.
Using hollow core fibers in RFOGs is of great interest for alleviating the aforementioned errors due to light propagation in glass within conventional fibers in RFOGs. In this case light is guided, via the optical band-gap effect down the center of an optical fiber with a hollow core so that the vast majority of light is traveling in free-space (vacuum or air, for instance), greatly reducing the rotation-equivalent error terms that stem from light propagation in a glass core fiber. However, one drawback that this invention addresses is that the fiber end is open to contamination of gases and particulates that may effect optical loss and optical properties of the fiber over time.